Said to fix PS Mini support
The release of the 1.65 firmware for the PS Vita was not without its problems. Sony pulled the firmware when it realized that it had an issue, and then later replaced it with the new version 1.66 that has now been released.
Version 1.66 is said to fix the issue that Vita users were having with the PS Mini games. In addition, users are saying that you should be able to get the older free Minis that you got when you subscribed. Just delete the existing, non-functional version and re-download it again, and the problem should be resolved.
Sony does not talk about the Mini fix in its release notes, but it does mention a few other tweaks that really are small things (like changes to the power save settings and Caps Lock now being supported). If you have PS Vita, you have likely already downloaded and installed it.